So-called mufflers are connected to exhaust pipes for reducing the noise of an exhaust gas discharged from a heat source such as an internal combustion engine in which the exhaust pressure widely varies. As shown in FIGS. 17 and 18, one conventional muffler is constructed of an outer muffler panel (91), filamentary glass fibers (92) attached to an inner surface of the outer muffler panel (91), and punched panels (93) held against inner surfaces of the glass fibers to hold the glass fibers. This construction is poor in heat insulation since the punched panels (93) which is a good thermal conductor and the outer muffler panel (91) are held in contact with each other through wide areas (94).
As shown in FIG. 19, there has been proposed a muffler for being mounted on a motorcycle engine which has a small displacement, the muffler comprising a silencer (96) made of glass fibers baked and pressed into a mat with an organic binder and attached to an inner surface of an outer panel (95). Since the binder is organic, the proposed muffler is of poor heat resistance. The heat resistance has been improved by employing an inorganic binder. However, the muffler is disadvantageous in that the silencer (96) has a low density of 150 Kg/m.sup.3, the glass fibers are joined together with small binding forces, the surfaces of the glass fibers of the silencer tend to split finely due to an exhaust gas flow, resulting in poor durability and an increased resistance to the exhaust gas flow thereby to impair the exhaust gas inertia.